The invention relates to a method of implanting dopants in a wide band gap material. More particularly, the invention relates to implanting dopants in such materials and repairing damage caused by implantation.
Wide band gap materials, such as gallium nitride (GaN), are well-suited for applications in various optoelectronic devices, including light emitting diodes, lasers, and ultraviolet radiation detectors, as well as for electronic devices operating at high temperatures, high frequencies, and high power levels. The viability of any optoelectronic or electronic device technology based on such materials depends on developing controlled doping techniques either during growth, or subsequently using diffusion or ion implantation. The incorporation of dopants by diffusion into many of these materials is very difficult, due to low diffusion coefficients of dopant elements. In addition, lateral doping during film growth is impossible for microcircuits. Thus, ion implantation is the only planar-selective area doping technique that is presently available for many wide band gap materials.
The main obstacle to achieving high quality implanted layers in many wide band gap materials is poor recovery of the radiation damage introduced during high dose ion implantation. This problem is of particular concern for doping these materials with acceptors and optically active dopants. The difficulty in p-type doping such materials by ion implantation is related to three major reasons. First, the high electron background concentration, caused by n-type impurities and native defects formed during growth of the wide band gap material, has to be overcompensated. Second, relatively deep acceptor levels result in only a small fraction of acceptors being ionized at room temperature. Third, implantation-induced defects may compensate the acceptors and getter the implanted elements, thereby rendering them electrically inactive. The difficulty in doping with optically active dopants is primarily related to native and implantation-induced defects, which provide non-radiative recombination channels.
Among the factors listed above, defects play a major role. N-type impurities and native defects can be removed by improving growth conditions of the wide band gap material. In the case of implantation-induced defects, an appropriate annealing procedure has to be employed. However, beginning with certain ion dose levels at which secondary defects such as dislocations, damage clusters, and amorphous zones start to form, radiation damage in such wide band gap materials is extremely stable against thermal annealing. To remove this damage, very high annealing temperatures (>1300° C. for GaN) are required. To inhibit thermal decomposition of materials such as GaN, annealing at such temperatures must be conducted at extremely high nitrogen pressures (˜10 kbar), or must utilize a cap layer (aluminum nitride (AlN), for example, serves as a cap for GaN). Annealing materials at such high temperature, however, create other problems. For example, hole density and luminescence intensity tend to decrease in GaN after high temperature annealing, and the AlN cap layers undergo local failure.
Other attempts to reduce the ion damage include ion implantation at elevated temperatures. In spite of observations of ion implantation damage reduction, no significant improvement in electrical properties has been reported. In addition, high temperature implantation results in other problems, such as reverse annealing, radiation enhanced diffusion, decomposition of the host material, and formation of very stable defects, the latter being more difficult to remove by annealing when compared to implantations performed at room temperature.
Repair and recovery from radiation damage incurred by implantation of dopants in wide band gap materials is currently difficult to achieve. Therefore, what is needed is a method of implanting high concentrations of such dopants while minimizing radiation damage. What is also needed is a method of repairing radiation damage caused by the implantation of such dopants.